Beforehand, some general considerations which allow a better understanding of the invention:
CO emissions of gas turbine engines need reductions for the sake of saving the environment. Such emissions are known to appear, when there is not sufficient time in the combustion chamber to ensure the CO to CO2 oxidation, and/or this oxidation is locally quenched due to contact with cold regions in the combustor. Since firing temperatures are smaller under part load conditions CO, and the CO to CO2 oxidation gets slower, thus CO emissions usually tend to increase under these conditions.
A reduction of CO emissions in turn might be invested in lowering the gas turbine load at the parking point of a gas turbine. This reduces the environmental impact due to reduced CO2 emissions and overall cost of electricity due to less fuel consumption during engine parking. Finally the CO emission reduction might be invested in a reduction of first costs due to savings on a CO catalyst. In this case a CO catalyst might be avoided (or at least reduced). At the same time losses, which appear due to a catalyst will be removed (or at least reduced), and thereby the overall efficiency of the power plant increased.
According to the US 2012/0017601 A1 the basic of this state of art is a method for operating the gas turbine, which keeps the air ratio λ (defined in US 2012/0017601 A1 paragraph 0009) of the operating burner of the second combustor below a maximum air ratio λmax during part load operation. This method is characterized essentially by three new elements and also by supplementing measures which can be implemented individually or in combination.
The maximum air ratio λmax in this case depends upon the CO emission limits which are to be observed, upon the design of the burner and of the combustor, and also upon the operating conditions, that is to say especially the burner inlet temperature.
The first element is a change in the principle of operation of the row of variable compressor inlet guide vanes, which allows the second combustor to be put into operation only at higher part load. Starting from no-load operation, the row of variable compressor inlet guide vanes is already opened while only the first combustor is in operation. This allows loading up to a higher relative load before the second combustor has to be put in operation. If the row of variable compressor inlet guide vanes is opened and the hot gas temperature or turbine inlet temperature of the high-pressure turbine has reached a limit, the second combustor is supplied with fuel.
In addition, the row of variable compressor inlet guide vanes is quickly closed. Closing of the row of variable compressor inlet guide vanes at constant turbine inlet temperature TIT of the high-pressure turbine, without countermeasures, would lead to a significant reduction of the relative power.
In order to avoid this power reduction, the fuel mass flow, which is introduced into the second combustor, can be increased. The minimum load at which the second combustor is put into operation and the minimum fuel flow into the second combustor are therefore significantly increased.
As a result, the minimum hot gas temperature of the second combustor is also increased, which reduces the air ratio λ and therefore reduces the CO emissions.
The second element for reducing the air ratio λ is a change in the principle of operation by increasing the turbine exhaust temperature of the high-pressure turbine TAT1 and/or the turbine exhaust temperature of the low-pressure turbine TAT2 during part load operation. This increase allows opening of the row of variable compressor inlet guide vanes to be shifted to a higher load point.
Conventionally, the maximum turbine exhaust temperature of the second turbine is determined for the full load case and the gas turbine and possibly the downstream waste heat boiler are designed in accordance with this temperature. This leads to the maximum hot gas temperature of the second turbine not being limited by the TIT2 (turbine inlet temperature of the second turbine) during part load operation with the row of variable compressor inlet guide vanes closed, but by the TAT2 (turbine exhaust temperature of the second turbine). Since at part load with at least one row of variable compressor inlet guide vanes closed the mass flow and therefore the pressure ratio across the turbine is reduced, the ratio of turbine inlet temperature to turbine exhaust temperature is also reduced.
Correspondingly, with constant TAT2 the TIT2 is also reduced and in most cases lies considerably below the full load value. A proposed slight increase of the TAT2 beyond the full load limit, typically within the order of magnitude of 10° C. to 30° C., admittedly leads to an increase of the TIT2, but this remains below the full load value and can practically be achieved without service life losses, or without significant service life losses. Adaptations in the design or in the choice of material do not become necessary or can be limited typically to the exhaust gas side. For increasing the TIT2, the hot gas temperature is increased, which is realized by an increase of the fuel mass flow and a reduction of the air ratio λ, which is associated therewith. The CO emissions are correspondingly reduced.
A further possibility for reducing the air ratio λ of the burner in operation is the deactivating of individual burners and redistribution of the fuel at constant TIT2.
In order to keep the TIT2 constant on average, the burner in operation has to be operated hotter in proportion to the number of deactivated burners. For this, the fuel feed is increased and therefore the local air ratio λ is reduced.
For an operation which is optimized for CO emissions, in a gas turbine with split line, a burner (for example for the second combustor) which is adjacent to the split line is typically deactivated first of all. In this case, the plane in which a casing is typically split into upper and lower halves is referred to as the split line. The respective casing halves are connected in the split line by a flange, for example.
Its adjacent burners are subsequently then deactivated or a burner, which is adjacent to the parting plane on the opposite side of the combustor is deactivated and in alternating sequence the adjacent burners, which alternate on the two sides of the combustor, starting from the parting plane, are deactivated.
A burner which is adjacent to the split line is preferably deactivated first of all since the split line of a gas turbine is typically not absolutely leak proof and in most cases a leakage flow leads to a slight cooling and dilution (see below mentioned considerations) of the flammable gases and therefore to locally increased CO emissions. As a result of deactivating the burners which are adjacent to the split line, these local CO emissions are avoided.
The combustion instabilities which are to be avoided by means of staging, typically no longer occur at low load or are negligibly small. In one exemplary embodiment, it is proposed, therefore, to carry out the restricting not by means of a fixed restrictor but by means of at least one control valve. This at least one control valve is opened at low load so that all the activated burners can be operated virtually homogenously with a low air ratio λ. At high load, the at least one control valve is throttled in order to realize the staging.
Referring to the currently proceeding cooling air from the reheat (second) combustor and any remaining air from the premix (first) combustor or fresh air from plenum are supplied as dilution air to the combustor(s) separately, as shown in FIG. 2.
In order to have sufficient backflow margin, both dilution air streams have to be injected with an excess pressure of about 1.5%. But the problem is, that not all flow paths are at the same pressure level, due to different pressure drop characteristics of sequential and premix liner cooling as depicted in the FIG. 2a. 
The configuration as shown in FIGS. 2/2a leads to at least 8% combustor pressure drop, as result of aforementioned backflow margin requirement and differences among all three flow paths. Pressure drop must be artificially increased in premix liner and premix burner circuits in order to match the sequential liner cooling circuit.